Is there an adjectival or adverbial form of "legacy"?

I think that the adjectival and, by extension, the adverbial form of legacy should be legatial and legatially respectively. This choice is based off the establishment of existential, gladiatorial, etc.

Words ending in the -ial suffix https://en.wiktionary.org/wiki/Category:English_words_suffixed_with_-ial

The -ial suffix is a very simple suffix of converting nouns into adjectives.

However, many words ending acy tend to change -cy to -te:

degeneracy->degenerate

celibacy->celibate

articulacy->articulate

Others change to -tic, because -cracy, meaning rule, is the suffix in these words:

bureaucracy->bureaucratic

autocracy->autocratic

aristocracy->aristocratic

Finally, there are a few that add the -ious suffix:

contumacy->contumacious

efficacy->efficacious

fallacy->fallacious

Finally, there is one that has, from what I can tell, no direct adjectival form (conspiracy) but whose closest adjective is conspiratorial, which comes more directly from conspirator. OED2 does have conspiratious (def: Addicted to conspiring) and conspirative (def: Engaged in, involving, or characterized by conspiracy).

http://www.morewords.com/ends-with/acy/

Now, that leaves legate, legacious, and legative, excluding legatial. Legate already exists as a noun with a different meaning, although it could be used as an adjective. Legative, too, exists as a an adjective derived from the previous legate. Legacious has no precedence as a word and could very well be coined for such an instance, and the -ly suffix can be easily slapped on to make legaciously.

'Legacy' is the adjectival form of 'legacy'.

This process orders entries in a legacy way.

If you're hip enough, you can even use 'legacy' as the adverbial form of 'legacy'.

This process orders entries legacy

Using "legacy" as an adjective is not "hip." It is indicative of shortcut laziness by millennials. The words "historical" or "discontinued" are more appropriate.