I havent messed with any of the betas, I'm incredibly careful when upgrading OS for my laptops. I kept my 2016 NTB on Sierra the whole time, and only have Catalina because my 2020 Air shipped with it. Though I was ready for some up to date ness since that one was becoming long in the tooth (though I appreciate 32-bit support still on my other machine, and there are certain things I just dont want to mess with, especially now that I have another machine).
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anyways, my tendencies aside are you enjoying Big Sur?
I'm ambivalent on the design elements. They're different enough to be different and for difference's sake. But not enough to make me feel like it's a whole new operating system (and for being 11, especially after a game changing 20 years of 10). I know it's important to Apple as it marks the first OS on their own chips, but it doesn't feel worthy of "11" so far.
That said, it really seems about as smooth and stable as Mojave was so far. It's definitely not the mess that Catalina is so far. I think that, as far as Catalina is concerned, it is already a massive improvement.
As for keeping 32-bit support, I highly recommend you at least move to Mojave. Much more stable than Sierra, in my opinion. Plus, you're gonna get security update support for another year.
Is it my imagination or did we have a transparent menubar before, and then it was decided against?
I'm not sure if this is a memory of an earlier version of MacOSX, or it might have been on Linux, in Gnome and/or Unity on Ubuntu, either way a few years ago now. This experiment has been tried before, and whichever platform it was on, they changed their mind and disabled it on the next release. Because it's OK as long as the background is co-operative (as with the default on Big Sur, of course), but with too many other backgrounds it just gets messy and indistinct if what's behind the menubar is too busy.
You're not mis-remembering it, pre-Yosemite (10.10.x) and dating as far back as Leopard (so, 10.5.0 to 10.9.5), the menu bar was SOMEWHAT translucent. Not to this degree, but definitely more than it was pre-Leopard and post-Mavericks.