sl-predicative
It's a term used in literature (eng. predicative, sl. povedkovnik) in reference to words such as všeč, žal, zima, etc. (see [1], [2], [3], where they're treated as a separate grammatical category) For the latter, it's also possible to analyze them independently of their occurence as predicatives and then describe when they occur as predicatives as merely "peculiar". Všeč doesn't appear in any other form except as a predicative. It's comparable to the Category:Russian_predicatives. You can change it to something else, I just wanted to add the word but I don't know of any other category where it would fit.
It's more that I'm not familiar with this word, and most other users of Wiktionary won't either. If it's a commonly accepted form then we should probably keep it, but we should find some way to describe what it actually means, maybe in Appendix:Glossary or something.
I'll ask in the Information Desk to see if anyone can help with this. You can join too if you want to.