For me, (and I think a lot of people on here) we initially focussed only on desktop when learning web dev bc it's the preferred platform and most tutorials just assume you're using it, then mobile dev is seen as an additional skill if you've got time for it afterwards. I think that's why mobile gets neglected, which sucks.
In my case, my site is definitely guilty of flexbox and javascript crimes bc I love to use all that magic stuff, but it's at a point where I don't have a clue how I could make a static version that works on all platforms with all the same features, or if it would even be possible. That's why I use the "best viewed on desktop" warning, it's more like an apology than a statement of disregard, like "sorry everyone, I wrote all this while testing on desktop and now I don't know how to convert it".
I think the best solution I can work on (something I know a lot of people already do and I'm super late to) would be to have a static, mobile-friendly version as default and then an option to switch on the dynamic stuff if the user is on a suitable platform.
I've definitely seen sites with stuff like "mobile-hostile" which does come off mean and unnecessary, but I think most people with desktop preferred messages aren't doing it spitefully. People just develop with desktop in mind because that's what they've been taught, then realise the features they implemented don't translate well to mobile.
We should try to change that though, and have more developers considering mobile platforms from the beggining so we can enjoy the accessibility and have fun dynamic features at the same time.