Instagram is indeed testing unskippable ad breaks that temporarily prevent users from scrolling, which has caused some user backlash and discussions about uninstalling the app.
According to the post of Agent Gray, Instagram feature has “un-skippable” ad break that ppl stop using the app and uninstall

Instagram testing controversial unskippable ads in its app
Source: 9to5mac.com
Key Evidence
Instagram confirmed testing unskippable ads with a countdown timer that blocks scrolling.
theverge.comtheverge.comInstagram is testing ‘unskippable’ ads that you can’t scroll past
techcrunch.comtechcrunch.comInstagram confirms test of 'unskippable' ads
Users on social media and forums have reported encountering these ads and expressed frustration, with some stating they might quit Instagram.
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The Verge and TechCrunch both cite Instagram's official confirmation of this feature.
theverge.comtheverge.comInstagram is testing ‘unskippable’ ads that you can’t scroll past
techcrunch.comtechcrunch.comInstagram confirms test of 'unskippable' ads
What the Evidence Shows
Multiple reputable sources confirm that Instagram began testing a new feature involving unskippable ad breaks in mid-2024. These ads force users to watch a short advertisement before they can continue scrolling through their feed.
The feature includes a countdown timer during which users cannot bypass the ad, effectively pausing their browsing experience.
This change has sparked negative reactions among users, with some expressing frustration and considering quitting or uninstalling the app due to the interruption in user experience.
Instagram spokesperson Matthew Tye has publicly confirmed the testing of these unskippable ads, lending credibility to the reports.
While the feature is still in testing and not universally rolled out, the claim that it exists and has caused some users to stop using Instagram is supported by current evidence.
However, it is important to note that widespread mass uninstallations have not been independently verified; user dissatisfaction is documented but may not represent a majority or large-scale exodus at this time.