I'm sure they purposely left Steve Rogers' whereabouts post-Endgame intentionally vague just so they have the possibility to bring him back later. Remember, Banner made a time machine that pushed time through people, so there's always the chance that he could do that to Steve and make him young again. Or perhaps if Chris Evans returns to the role, his next appearance could be from his life with Peggy, or from an alternate timeline/future/past.
Either way, it's a loose end that, hopefully, will get tied up some how in a nice neat bow, but I won't hold my breath. The MCU can be pretty good about its continuity, but there are always a few loose ends that are never address again. I'm still amazed at the fact that over a decade later, we will finally revisit Emil Blonsky/Abomination in the upcoming She-Hulk series (even though I would have rather seen The Leader finally make his big-headed appearance in the MCU).
-Marc.