Derailleur compatibility with Shimano 5500 shifters Working on an old bike with a hodgepodge of groupset components - 5500 "105 flight deck" shifters (apparently 9 speed from what I've been able to work out online), a 3x Sora crankset, and a 9 speed cassette in the rear. The rear derailleur was an old Ultegra whose outer cage plate literally snapped off while cranking on a particularly steep incline (the thing was literally caked in the nastiest grime I've ever seen on a bike for years - not my doing, just started riding it a month ago). I would ideally like to replace the derailleur with an unused component but am having a hard time figuring out compatibility with this system - I can't find any rear derailleur corresponding to the 5500 series via google searches and the more modern 105 options are all either 10 or 11 speed. I'm new to the bike repair game and so far have come to understand that my main restriction is the number of speeds has to match between the shifters and the derailleur, but I've also seen some sources say that the cable tension standards for groupsets can change in big generational overhauls. 

Ultimately, I'm first and foremost looking for any suggestions regarding compatible derailleurs for this bike as well as some pointers on what to look out for when considering component compatibility in the future.
You have many options for a replacement. Essentially any pre-11-speed Shimano road derailleur with a GS cage length will work, excluding Tiagra 4700 and pre-indexing/early indexing ones.

The rear derailleur actuation ratio is the same across many generations of Shimano groups. The earliest Dura Ace indexing groups used one ratio, now a footnote in history, then everything else used the same ratio (known unofficially as 1.7 in some sources) for 6, 7, 8, and 9 speed groups. Road 10 also used 1.7. The first break from this compatibility came with Mountain 10, and since then every speed generation has been its own thing, with the exception that Tiagra 4700 is a ten speed road group that uses the 11-speed actuation ratio. (There will probably be more like this going forward.)

For the most part, the ten speed era derailleurs that use the same actuation as your 9-speed-era Ultegra will drop in and work with zero issues. This includes any GS derailleurs from 5600, 5700, 6600, 6700, 7800, and 7900. You need GS because you have a triple. You could also use one from the 9-speed groups, like RD-5501-GS, etc. (That's the one that "matches" your shifters.)

It's an old group now but I believe Shimano is still producing RD-5701-GS as a legacy option for a high quality replacement for the compatibility you need. That matches the quality of the rest of the bike so it probably makes some sense to just get that one. RD-R3000-GS would be a functional budget choice. After what the bike has been through, make sure the hanger is aligned. It's also prudent most of the time to start out with at least a new cable and ideally new cable and housing with a new RD.

Using a less-than-9-speed derailleur with the correct actuation can work, but should usually be avoided because the precision might not be there for it to perform well.