It is slightly arhaic but easily understood.
You see, with words like "лицеприятие" the problem is, there is no clear meaning. "-ие" kinda signals that the word means some phenomenon or process. However, no words look similar to that one. You have "нелицеприятный" which is now mostly used as "unpleasant" except by people who checked it in the dictionary.. sooo.. is "лицеприятие" a word for "pleasantition"? "plesantity"? "pleasanation"? With the old meaning of "нелицеприятный"(unprejudiced) it was obvious what the meaning is, but not anymore in the lack of proper context.
You have a completely different picture with "заповедовать/заповедать". Russian still has "заповедь" (commandment) used in religious contexts and, figuratively, in some others. Russian still uses the root "вед" (as in "поведать", "в неведении", "без моего ведома") and "заповедь" actually was formed as "the things that are told". So a native speaker may not be sure such a word as "заповедовать" exists but assuming it is actually a Russian word, one can easily see what is its most probable meaning.
Actually, there are two words in modern Russian that should be known to almost any native speaker older than 9 or 10 years:
- заповедь = commandment (religious)
- заповедник = nature reserve
They are beyond the list of the most frequent 5000 words, but clearly in the most used 10000–15000 — so yeah, even a moderately well-read elementary school pupil has enough passive vocabulary to make a good guess what "заповедовать" means. The good thing is, this guess is correct.