@burnout426 thanks, that Reddit thread helped, most of the data is recovered, except the tabs, but I guess that'll be a matter of playing around with the Session folder, which I backed up as soon as this happened.
Edit: yeah, tabs recovered now too, had to remove newer Tabs_ and Session_ files first.

I can't believe such a big and widely used product messes up something as important as data migration and it's not hotfixed yet as I see it started happening around a week ago.

As for autostart, that doesn't help. The issue is that the installer for some reason has adding autostart entry enabled by default. I worked around it by adding a parameter to the package manager I'm using - chocolatey. But even after that, which worked for a bit, some kind of internal autoupdate of Opera readded the entry... It should be disabled by default in the installer in the 1st place IMO. Because who wants to autostart their browser with system, realistically? It's not good for performance (especially with more tabs than my 30-40), doesn't have reasonable advantages and you can turn it back on with a matter of a click if it's pinned to taskbar.

Web browser is not something that you normally want launched at the start of the system. It's really bad for optimal startup, especially considering that some users have hundreds of tabs opened.

Advantages? Almost none. You save 1 click compared to pinning it to taskbar and opening it manually after everything starts.

Thereby I don't know why the default is to add the registry entry, including during each upgrade and autoupgrade (!)

Any insights on this?

Web browser is not something that you normally want launched at the start of the system. It's really bad for optimal startup, especially considering that some users have hundreds of tabs opened.

Advantages? Almost none. You save 1 click compared to pinning it to taskbar and opening it manually after everything starts.

Thereby I don't know why the default is to add the registry entry, including during each upgrade and autoupgrade (!)

Actually, cookies were not recovered. Is it just the Cookies file in Network folder or something else I can try copying from old files? Not the biggest deal for me since I'm using a password manager anyways, but it'd save some time if I didn't have to log in to like 50 websites again.

The rest I can confirm as recovered.

@burnout426 thanks, that Reddit thread helped, most of the data is recovered, except the tabs, but I guess that'll be a matter of playing around with the Session folder, which I backed up as soon as this happened.
Edit: yeah, tabs recovered now too, had to remove newer Tabs_ and Session_ files first.

I can't believe such a big and widely used product messes up something as important as data migration and it's not hotfixed yet as I see it started happening around a week ago.

As for autostart, that doesn't help. The issue is that the installer for some reason has adding autostart entry enabled by default. I worked around it by adding a parameter to the package manager I'm using - chocolatey. But even after that, which worked for a bit, some kind of internal autoupdate of Opera readded the entry... It should be disabled by default in the installer in the 1st place IMO. Because who wants to autostart their browser with system, realistically? It's not good for performance (especially with more tabs than my 30-40), doesn't have reasonable advantages and you can turn it back on with a matter of a click if it's pinned to taskbar.

Opera keeps adding itself to autostart which is beyond annoying by itself, as it also happens when it autoupdates itself. I have no clue who thought this installation default is a good idea.

However, today it resulted in autostart launch into it saying that it updated to Opera One (but it has already been on that new UI for a while). I don't see any history, extensions, cache, etc. I also lost 30-40 tabs that were open.

It says I can log in to sync, but I'm not doing anything for now to have a higher chance of recovery. Will logging in bring everything including old tabs back? Or what else can I do?There's no file backup unless Opera backed something up itself.

My BT headset (Momentum M3) has a feature that simulates media pause/start controls when you take your headphones off. It works not perfectly and sometimes pauses the video when you move your head to a weird position. Which is fine in e.g. Chrome but not in Opera when the app is in picture in picture mode. The audio often gets disabled then and the video doesn't stop. The unpause functionality doesn't work either. So the video just continues to run silently in the background, which is annoying when you're far from laptop.

Any idea how to get around that? Or is that just a bug/bad implementation of media controls?

So, this looks to be a bug of some random version that I upgraded back then. It doesn't happen anymore, I tried few upgrades and the autorun entry isn't added. Unless it's something entirely else that added it.

@leocg thanks

Is there any list of CLI paramaters/switches for Windows installer?

I can't find them documented anywhere.

@leocg Ok. One more question, is there an installer CLI switch to turn it off? Or is there any list of CLI parameters for the installer?