Office 2011 has a few problems on High Sierra. There are also some problems with some versions of Adobe CS on High Sierra.
I'm still using Office 2011 for Mac version 14.7.7 on all five of my Macs and all are running High Sierra 10.13.6. I haven't seen any issues at all but maybe there are some. I read that some of the data integration features such as with Calendar (which I don't use) require Office 2016 for Mac or later (Office 2019 for Mac is on the way) but that has been an issue for several years. I would make sure Office 2011 is fully updated to the latest 14.7.7 before upgrading the MacOS.