RobTop confirmed that in Geometry Dash 2.2, the upcoming platformer mode will be timed. Once the player runs out of time, they will need to retry the level.
RobTop failed to specify whether or not an "unlimited" time option would be available to creators.
There are also options to increase and decrease the time remaining by collecting collectibles. These collectibles can be set by the creator.
This will be added in the Geometry Dash 2.2 update which currently has an expected release date of this year.
This was all confirmed in the official Geometry Dash Discord server.