Yes, thankyou, that's quite obvious judging by the quality of most software.
It really is amazing how bad most software made for non-developers is. Like, as software engineers, we understand how essential version control is. We made git and github for ourselves. But nobody has bothered building that functionality for people who edit word documents all day. Or people who edit video, or animators, or 3d modellers, or 100 different jobs. Word and google docs have track changes. But they don't let you bounce between branches or make pull requests. You usually can't time travel, or bisect, or git blame, or any of the other things we take for granted. My partner works in a CMS all day at work. Every change she makes is pushed directly to production. There's no review process. No staging. No testing. No change control or rollback. If anyone messes something up, they get blamed for "taking down the app". As a software engineer, I look on in horror.
I believe the more cognitive distance there is between 20-something silicon valley tech bros and your particular use case, the worse your software is going to be. If you're a manchild living in san francisco who can't be bothered driving, doing your laundry or shopping for groceries, you're in good hands. There is a startup that will solve your problem! But the further from that "ideal" you get, the worse. Here in Melbourne, I can't use my iphone to pay for public transit. Google maps couldn't really handle roundabouts (traffic circles) for a decade and change. (I guess they don't have those in California). Unicode support was only added recently because of Emoji. Until then, a huge amount of software butchered non-english text. I shudder to think how badly most software probably handles right to left languages. And the list goes on and on.
Fwiw that just sounds like an immature CMS - I've seen review/approval workflows, branches, preview environments etc in more than one CMS. I take your overall point but maybe your partner doesn't have to live this way.
Nested H1s was never semantically correct in the first place, at least for accessibility purposes.
You can do flexible sizes without media queries (eg, viewport size units + clamp). Designers generally understand the web pretty well these days.
I only see one situation where people might have depended on these styles, but it's a big one - anywhere that you output the plain HTML of a "rich text" component from a CMS or whatever. There, if the stakes are low, it might not have been a big deal to just let the browser do it and headings might look too big sometimes now.