I added a guideline to an uppercase glyph and then I applied a filter so that it is displayed in uppercase letters only. The guideline disappeared. I discovered there is no information in the Subcategory column for most of the glyphs in the file. Is it a bug?
Case is no longer defined in the subcategory, but separately. So applying the filter like this should work:
But perhaps we should be able to sort by case in the List view? 
Ok, good to know! I did not see this new item in the menu! Thank you!
I guess so, otherwise it seems a bit confusing to me.
Beside that, I think the Subcategory tag should be kept for Latin uppercase and lowercase letters. There are some scripts and plugins (e.g HTLetterSpacer) that won’t work properly without that tag. I added it myself editing the GlyphData.xml file. HTLettersSpacer works as expected now. Wondering whether this may have unexpected consequences later… @GeorgSeifert @mekkablue
There are a lot places where the subcategory is not sufficient. E.g. with Ligatures you can’t specify the case.
And it should be easy to fix those plugins instead of hacking the GlyphData.
Discovered a couple of weeks ago that Letterspacer works as expected in Glyphs 3. (No need to hack the GlyphData) The problem was that I was reusing a config file from Glyphs 2. Of course, it did not address the “case” tag since it did not exist in Glyphs 2 (see images below)
Glyphs 2 config file caption
Glyphs 3 config file caption
Sorry for the mix-up.