They exist, you just have to look at specs of ones that you like. You can also get an analog to bluetooth adapter and vice versa. You don’t say whether you want a big speaker or a small one, or what your budget is. That could help. I guess the adapters need power, which isn’t so great for portability.

I know that power tool companies make some job site speakers with aux in, that run on tool batteries. They are on the large side though.

I just have a dinky little one, idk what brand, but it was cheap and suffices for personal listening, and has both 3.5mm and bluetooth inputs. It has an internal battery so I guess I’ll have to dismantle it to replace the battery someday. Be careful of internal batteries regarding BIFL. I know some of the older bigger ones used 18650’s inside and that was attractive. I didn’t buy one though.

My phone has analog output (I made sure to choose a phone that has that). In principle it should be possible to forward bluetooth audio in to analog audio out, or route the bluetooth input to the phone’s internal speaker, but I haven’t looked for an F-droid app that does this.

Keyword to look for in portable speakers is “aux”, referring to the analog input. A quick search just found this, which looks sort of cool and BIFL, if the 2600mah battery capacity refers to an 18650 inside:

https://www.bhphotovideo.com/c/product/1750016-REG/sangean_wr_7wl_wood_cabinet_mini_bluetooth.html/specs