Will DEL (Delhi airport) ever be a playable airport

Short Answer: Possibly in the future.

The developers announced DEL as a future playable airport early in 2023. However after that point. Unity, the company that provides the platform and coding structure for the game changed their licensing policies and pricing to a per download model.
While many airports such as LHR and SYD (coming soon) would still have this issue, the developers could be able to make enough revenue from Gold Plane Sales with them to offset this cost.

For DEL, the number of downloads would likely be enormous, and the potential game revenue against that would also likely not be enough to cover the download costs, thus it would not make it cost effective and risk work elsewhere.

As a result, at time of writing (3/15/2024) DEL is on an indefinite hold pending either a change in the licensing structure from Unity or the ability to find a way to make the economics. DEL is on INDEFINITE HOLD (meaning it may happen, but no plans for it to happen at this point)