I'm looking for anything in spring 21 that could cause trouble in my existing instance. So far while going through the release notes I only see enhancements that I do not anticipate causing problems. Does anyone know of anything that could possibly cause issues?
The primary concern for most people are the removal of permissions from the Guest User profiles. If you use this profile for public features (e.g. Sites, Communities), make sure that these features still work. If you do nothing, you may find them to be broken.
In addition, note that BEM notation for SLDS styles are being deprecated, so now is the time to search-and-replace those
-- characters with
_ in your SLDS styles for any Lightning Components and Visualforce Pages that use SLDS. They won't stop working in Spring '21, but they will stop working in Summer '21. This won't cause performance problems, but will certainly cause graphical glitches in your components.
Aside from these two features, almost everything else is either an enhancement that you can take advantage of, or a feature you need to enable specifically. There's not too much you need to worry about. I haven't heard of any other major concerns regarding this release, although I have not had the time to read all the content.
As is true for any release, you can look through any known issues tagged to the release ahead of time. Many will be patched before release into production, but can be useful to look at to prepare for things that may not get patched until after the initial release.
I am leaving this as a Community Wiki so that others may add any additional findings I may have missed.