Since you asked, it's not so much that Apple wants you to constantly buy new Macs (although I suppose they wouldn't mind if you did), the fact is Apple considers operating system security an essential part of their core business model. Their single-minded approach to security extends to protecting your own personal information, which in turn makes it essential Safari lives up to their reputation as well. In practice, it means that every website, every developer, every downloadable product must absolutely, positively comply with whatever the latest security practices happen to be.
In the case of truthsocial, there is nothing to stop its developers from supporting older versions of Safari. In fact if you're an eBay or Amazon or whatever, they're motivated to attract as broad a consumer base as they can, because they are less concerned about how they derive their revenue. Both those sites load just fine on geriatric Macs.
If I had to guess though, Truth Social is justifiably concerned about bad actors spoofing their website or doing any number of nefarious deeds in an effort to shut them down, so they are going to be motivated to use as much platform security as technologically possible. But that's only a guess — since you asked.
At this late stage, 32-bit apps are really quite old. Apple announced the transition a long, long time ago, and only recently shut off support for them completely. If you need to run them though, you can certainly install a commensurately older version of macOS on an external hard disk drive, and boot from it when you need to run those apps. That's a common and relatively inexpensive workaround.
Safari gets updated along with macOS, so whatever version you are using is already appropriate for macOS 10.14.6.