Now, I also realize most of the folks here are not gonna be the ones to do this, BUT you might be the one to overhear it and just might be the one that can shift the narrative and make someone else feel more comfortable. Also, this post is NOT the place to drag out horror stories from a decade ago, but I would happy to hear about those times that something inspirational happened, or of moments of exemplary courtesy.
1) There is no starting date for the SCA. Period. Doesn't matter what you thought it was or what someone told you it was 20 years ago (or even yesterday). It is in the rules. There is no starting date. And, in case anyone is really wondering, this rule goes WAY back. Yes, at one point the handbook listed 600CE, but even then the rule still said pre-17th Century. Also, we have had very Early period Laurels for a very, very long time, so clearly, these efforts have been officially recognized for a long time as well.
2) We are not Eurocentric. Yes, at one time we were (or tried to be), but we aren't now. And as with the above, even before the rules were re-worded to be more inclusive, there were still folks being recognized for their spectacular efforts in these areas. There is a whole world of incredible history that can be explored, and we should welcome that as a good thing.
3) Of members we expect basic courtesy towards others and an attempt at pre-17th Century garb. Key word here is "attempt". Someone might be new, or they might be at a different stage in their journey, or they just might be comfortable dressing a certain way. You do not get to define if an attempt is worthy or not unless someone asks you about it. Likewise, courtesy means that you also do not get too negate a conversation about very deep historic interpretations among those who wish to discuss such things.
4) Just because someone is decked out in splendid periodocity doesn't mean that they are looking down with you. That is their work and passion and they have a right to display it and talk about it. You could learn something from them, or they could learn something from you, even if it is not about garb at all. We all have our passions and are similar in our love of sharing the things that inspire us.
5) That item #3 is specifically about GARB. It has nothing to do with the the project someone is sitting and working on at an event. Is someone knitting a day-glow 1960's inspired sweater at an event? Good for them! They are crafting and that is amazing. Guess what? They know it isn't period, and it's not your business to point that out. Let them do them because I guarantee you that that if that sweater that someone is quietly knitting on the sidelines ruins your day, you need help from outside the SCA and should go seek it.
5A) If a kid is wearing a Hogwarts robe or a Disney Princess costume, just step down totally. It is hard to get kids to do anything sometimes and if that makes them happy, then the parents are happy, and, likely, everyone in the vicinity is happy.
6) There are modern things that are necessary for medical reasons... shoes and eyewear are the two most common. Someone might be in a glorious Elizabethan outfit but is wearing sneakers because that is the only way they can stay on their feet all weekend. Your comments are really not needed, and they are very likely already aware of it and feel self-conscious.
7) And while many do not like it, phones are everywhere and will be everywhere. Courtesy demands that we silence them during classes and courts, and if would be kind if we would look at them a bit covertly during those times (unless we are taking a photograph), but this _is_ the world we live in and those phones allow parents to keep track of children, or allow us to in touch with folks offsite. You never know what is going on in someone's life so, please, don't judge the person that frequently is checking their phone. Also, phones are great tools to keep kids quiet sometimes, and as with number 5, that is often in everyone's best interest.
8) There are also people who work very, very hard on the more actual reenactment end of what we do. You do not have to do go that route, but in their own groups or own spaces, they have that right. These folks love to engage with others, teach others and are often great resources to all of us even if we are just dipping our toes in the water rather than diving in the way they have. This would include things like battles at some events that have kits required for a specific time and place. Individuals are working very hard to create a setting to try to portray a specific thing. If you do not have the gear for that, there are other fights for you. And I bet if you opt to watch it, you might be inspired and most likely folks would give you information to work towards participating in the future. Another example is a band that uses only period instruments or that only performs one type of music. If you are not equipped to play with them, you can still enjoy their work and learn from them. Yet another example would be Enchanted Ground at Pennsic, these folks welcome you into their home and have a set of rules for behavior when you enter (no modernity). You can walk in and join in and be welcomed, or you can choose to do something else with your time. There are always options of other activities if you do not care for the thing before you.
9) The most basic premise is that we should all be having fun. Try to help others with their own brand of fun when possible, and share yours with those who express interest. Volunteer to work on some facet of an event if possible, as burn-out among the regulars is real. If you see someone looking lost, maybe you can guide them to something they would like or you just might be the person who makes them feel included.