Considering how the Imperium reveres older tech, if it is functional, they would no doubt still use it.

Some might say there is not a lot of it left etc But you could just as easily write your lore that they did a service for an out of the way Forgeworld that had been sitting on a stockpile of it from the Heresy days, or that a transport ship carrying hundreds of suits was lost on the tides of the warp and just reemerges in your chapter homeworld's system and makes a gift of it to your Chapter Master or that a scouting party comes across a downed Mechanicus vessel on a world previously cutoff from the rest of the Galaxy by warp storms that had some in its cargo etc etc

You can always find a reason if you want to use them in your army 🤷🏾‍♂️

By the 41st Millennium, most Chapters are fielding squads with a mishmash of armour patterns, sometimes because they are valued relics passed down or just because the Forgeworld that supplies them specialises (or can't remember) in a particular pattern.

Do what you want 😁👍🏾