YouTube has been experimenting with different ways of displaying ads recently, such as non-skippable ads and ads that appear when a user pauses a video. It’s also been cracking down on ad blockers and “clone” apps. Ads help keep most of YouTube’s content “free” to users.
For many users, waiting a few seconds and then hitting "Skip" on an ad isn't a big deal because they know when they can skip it thanks to the countdown.
But recently, Youtube has just implemented a change in the ad skipping section on both the desktop and mobile versions. That is, the countdown timer to skip the ad is no longer there, instead there is a blank space or a gray rectangle on the computer version. In fact, the “Skip” button will appear after a certain period of time, but because the countdown timer no longer appears, users will think that the ad cannot be skipped, so they will stay on the ad longer.

This new move by YouTube is said to be aimed at optimizing advertising effectiveness.
This new change has been rolled out to some users on both platforms by YouTube. However, YouTube has not yet made any official announcement about this change, but it can be predicted that this will be a permanent change in the future.