Timeline seems to have been of rather niche appeal to most people.
Occasionally I would read a story where a journalist had stumbled upon it and was horrified to find an accessible window into the location data which is undeniably being collected by anyone who can (Apple Google Microsoft and many others I'm sure).
However for a few, at a guess 2-3% of phone users, it became incredibly useful. I was one, and my specific use was to keep track of vehicle mileage for tax reasons, which seemed to be a common use. Others myself included, used it to just see where they'd been generally.
The argument that a free service provided by someone else may be dropped at any time is true, and to rely on it for any matter of importance would be a mistake. But like the others in small minority who enjoyed the desktop browser and let it into their lives, I will greatly miss it's demise
There are a nother couple of points I'd like to mention.
Firstly, when following the Google instructions to move the timeline data to your phone, stop. Instead, before going any further, visit Google Takeout and download all your timeline data. Twice if you're paranoid. This is because the data Google sends to your phone has been heavily abbreviated, .and as per article above, some stuff seems to have been deleted. My own experience is that location points have been reduced or decimated by around 75%. If you keep the original Google Takeout files you will be able to import them into other solutions later.
Secondly, I wonder if this might have been an own goal on Google's part. Sure the service was niche, and possibly the data storage requirements (volume, regulatory etc) didn't seem attractive anymore, but they may have just lost the most useful volunteer map correcting team they didn't know they had. I wonder how many other people spend time correcting location physical positions, adding opening hours, answering a few questions about businesses' services and so on - perhaps this may show in time.
Lastly, next time - self host, back up and if it's worth doing, learn how to do it yourself. No one else will care as much as you.