All sig servers became COTS linux sig servers. The ISP 1100 sig servers aren't supported anymore, and thye earned their right to retirement after being in place since release 3.0.
We used the latest sig server and core deplists with no major issues to report.
UCM stands for Unified Communications Management. Its essentially a centralized login used to access element manager, the NRS manager, etc. I don't see a lot of benefit to it other than being another layer in the middle to get to. One nice piece is the centralized authentication which gives you single signon to all the elements in the security domain. UCM is handy, but in my opinion it should be optional, not mandatory.