Aren't you grasping at straws? How many times do you believe a project changes it's ticketing system? So far I saw that happening a grand total of zero times.
Meanwhile, I've repeatedly worked on legacy projects which have decades-old TODO/FIXIT items, which serve no purpose other than being noise and serving as topic in water-cooler shit chat.
Your company should get it's shit together, unless it's in the business of switching ticketing systems.
Though one other reason to migrate is the forms are complex and full of required fields nobody understands. Migration will get you past that to what matters, but only for a few years before the groups that required those fields in the first place come back and demand it with the same good but forgotten reasoning as the first time. If this is you, get your act together.
That is being generous.
Nobody was reading, triaging, or fixing them anyway.
Sometimes comments are useless crap I can deduce from the code but often I actually get some valuable information.
I won't track down a ticket containing the information unless I'm really desperate.
Unfortunately we live in an agile world and there is not a single doc worth its salt.