I used to be really into photography, starting with 35mm then moving on to the Rebel XT when it first came out all those years ago. Then life got in the way and I haven't pulled the XT out for at least 10 years. I am now looking to get back into photography, and I am looking at the M50. I still have a decent collection of lenses and flashes and so on from the XT and before, am I right in my understanding that with an adaptor they'll still fit? Or am I better off getting a 250D? Or something else?
All M-Series bodies require an EF.EF-M mount adapter to use EOS EF or EOS EF-S mount lenses.
Lenses made for the EF-M mount tend to be noticeably smaller than EF/EF-S mount lenses, which means they will draw far less battery power. I think both the SL3 and the M50 are entry level cameras, which are not as full featured as your XT was in its' day. Both cameras have small bodies and small batteries.
I recommend looking at the Canon Online Refurbished Store for the best deals on Canon cameras and lenses.
The T7i or the 77D are probably the cameras that you may want look at. The flash shoes on the M50 and SL3 may or may not be as full featured to work with your current flash units. I read many posts in the forums about people complaiining about them.
The Refurbished Store has items that go in and out of stock. Graduation season is coming up. I expect them store to bring a number of low cost items back into stock. The T6 and T7 are very similar to your XT, which means they are mostly old tech.
"I right in my understanding that with an adaptor they'll still fit?"
The definition of adapter is, it adapts something that was not designed to work on a gadget so it will work. Almost always it doesn't work as well as something that is totally engineered to work properly in the first place. Keep in mind there is a huge differnece between "fit" and "worK".
IMHO, you need to buy a T7i and use all your gear as it was designed. You could possibly run into some outdated tech, though. Not knowing exactly what all you have.