If it's not necessarily old, but not used now or recently, use "obs". "obsc" is more for current but rare terms.

@@ -12 +12 @@
-<misc>&obsc;</misc>
+<misc>&obs;</misc>
@@ -17 +17 @@
-<misc>&obsc;</misc>
+<misc>&obs;</misc>

daijr, daijs, nikk: https://kotobank.jp/word/饌-549233
the kokogos record only the せん reading but aozora indicates that it's also read ぜん:
https://furigana.info/w/饌

By the way, I still find it difficult to distinguish arch, obs, and obsc after half a year's contributing:

1. Not sure if 饌 is primarily used during or before the Edo period. To determine that, you need the unabridged edition of Nikkoku
   or subscription to the historical corpora at NINJAL to see the whole history. But since 饌 is a kango, it is likely to be used
   throughout the history of 文語 instead of being an 江戸語, so it is probably not arch.
2. It is used by Meiji and early-Showa period authors like Mori Ogai (as shown by aozora), but no longer in general use (ghits are few). So it is obs.
3. It is "rarely used" in post-WWII 口語, but not "rarely used" throughout history (as shown by aozora) -- the criterion for "obsc" is ambiguous.
   However, there are more common terms (供え物, 神饌, 献饌, 食饌, etc.), so it appears to be obsc.