Ironically the dictionaries you mention have greatly stabilised language. Language is going to be far more stable over the next 500 years. I wouldn't worry about it in the slightest.
I don't see much stability.
Many, many people speaking English also creates an incentive for stability at the same time it creates variation.
My lived experience is I can read texts from hundreds of years ago just fine. Old mate is struggling with 50s tv.
The only tip is to use stable language. IE don't use the term dab or vibe or whatever the latest fad niche speak is.
I think whoever is interested in a 500 year old website is going to be ok.
One is a 'phase' the other is cross generational.
Stuff like that can easily be looked up though, there are a number of "youth/hipster speak" dictionaires, pages like KnowYourMeme...