Well, significantly overdue as your post is 9 months old at the time of this post, but can view it as giving the changes (and nerfs to midlane malz that occured since mid season 7 through current season 8.3 patch) more "time" than your "give it a week" statement.
Malzahar completely fell of the support (and jungle) playrates on prolol and champion.gg and op.gg. Players can still do it, just like you could choose to play any champion anyway you want (AP Kog mid post nerf, etc), but when the lane has completely disappeared from the tracked lane stats for months, it indicates its been nerfed to the point of castration and obviously no longer meta (for support, not for mid lane).
At the time of this post: malz is shown only tracked Middle Lane on champion.gg and op.gg with an 86.23% rate, and no support or jungle tab, which at some past points he (briefly) did have when they were able to be done and considered viable / acceptable / meta. As for that remaining 13.77%, champion.gg tosses out games that end relatively early or have too few items on the champ to categorize to a lane with a worthwhile game, so those games are tossed/ignored, regardless if they were "remaked", surrendered at 15/20 or even a hyper win within those time frames.
As for the other guys Zyra comment, yeah she's still support without the mid lane sustain dmg she had back in s3, but now ( 9 months later) that her plants don't auto target champs on spawn (unless the spell that grows the plants hits a champ), + a follow up nerf increasing mana costs, before considering other changes to other champs and items, the winrate plummeted for her. And despite that being 9 months, I know, because I went Zyra first 1/2 of season 7 when i Saw she was consistently in the top 3 champ winrates end of s6 and stuck with her...until these nerfs which resulted in me gradually dropping her in favor of other champs that came up to god tier winrates at the time (Fiddle was one who became a top winrate when Rakan and Xayah were released and is still flagged even now as a high winrate mid and support).
As evident on the stats before and after on the playrate/winrate sites for both Zyra and Malz, it was obvious the playrates were going to drop and those roles were getting neutered. Same for zyra for the nerf "completely reworking the plant mechanic" in a huge manner no longer prioritizing champs on spawn. Didn't need "a week" to see either set playout. Thats why data on the playsites from the...what....33 million player poolsize? already shows data after 2 days of each patch.
9 months ago was only when the first couple nerfs to Malz's voidlings were made. But within those few months, the voidlings got literal mini-reworks 3x in addition to riot "buffing his mid lane ap damage" in the patch notes as compensation for nerfing the voidlings.
They went from spawning rapidly in groups with w growing into a clusterfuck of voidlings, into becoming the charged skill giving extras when you cast other spells, followed by the 3rd nerf slowing their movement speed upon spawn, then finally the 4th major voidling nerf after that where all voidlings die with 1 hit. And thats before you add the follow up nerfs to his ult cd and dmg at that time, which got nerfed again recently (though currently being s8 is a different world).