This is not a question on which thread to use when/where but a question on age of thread.
I've heard not to use "old" thread. I've bought thread "new" at stores and it unravels, leaves a lot of lint, doesn't sew right or breaks continuously. (Yes, I had it threaded correctly.) And I don't buy inexpensive thread.
As soon as I changed the tread, my machine (a Baby Lock) worked perfectly. This could have been the same color, same thread, bought at the same time! When I put the "bad" spool back on, same problems.

How can you tell if thread is too old (or just bad)? How old is too old?

i guess you can't tell a "bad" spool until it's not working right. on "old" thread, if you pull it and it breaks easily, then toss it.

I use vintage thread, but good brands on my Janome 10000 that I find in yard sales and at auctions. Most are cotton wrapped polyester. Some are 100% cotton. I am not a purist, and have a limited budget so being frugal is important to me. I test 100% cotton thread, by wrapping a length around my fingers (like I do for flossing my teeth), and give a yank. If the thread breaks very easily, I don't buy it or use it. The quilter's that came before us, used what was available, today some quilter's are true purist. That is not me.