Probably nothing - to have made that size means its done well anyway. You might just inspect the trunk and branches, checking for any signs of disease (weeping or soggy areas) but otherwise, see how well it crops this year, and whether the plums are good or display problems. The trouble is, if its 30 feet, any fruit will require a ladder to collect, so you might want to think about some renovation pruning - this link here (scroll down) gives information on how and when to do that

https://www.rhs.org.uk/advice/profile?PID=339

Yes, I was going to say similar...it might benefit from a good prune, but only during summer months. I inherited similar here, and it produced lots of fruit that never ripened. But this year...2 years after cutting it back hard and thinning the branches, We had a lovely crop of sweet juicy plums. The other thing I learned is that plum trees do need a good amount of water to ripen their fruit. So be sure to water it during summer if it gets very dry. Ours is at the top of a bank, so this can be a problem. Not this year though, as the summer here was very wet and cold.