Although probably intended to target large platforms, the OSA is worded in such a way as to apply to nearly everyone who provides any kind of service that allows user-to-user interaction. It is (rightly) ostensibly intended to prevent abuse, terrorism, grooming, and similar activities, but it requires a full risk assessment against around 10 different possible interactions, and the paperwork is potentially onerous.

But worse, the fines are potentially in the tens of millions of pounds, and even inadvertent non-compliance can result in extensive prison time.

Of course, the government says they won't go after the small organisations, but if the wind changes, the legislation is there. Even if the risk is extremely small, the penalties are life-destroying.

As a result, many are deciding it's simply not worth the risk.

Also explained here:

https://news.ycombinator.com/item?id=43152178

https://news.ycombinator.com/item?id=43152154

This comment is especially useful:

https://news.ycombinator.com/item?id=43152930

    Liability is unlimited and there's no provision in law
    for being a single person or small group of volunteers.
    You'll be held to the same standards as a behemoth with
    full time lawyers.

OSA is a censorship law that pushes the cost of monitoring and compliance onto website operators.

This means that unless you are a gigantic corporation with deep pockets and can afford all the hundreds of hours of lawyers, developers, and moderators needed to comply with the law then you really can't afford to operate websites that other people can use.

It is effectively handing over the keys of WWW to mega corporations in the UK.

If you are operating a personal website it doesn't impact you, but if you are doing forums or anything were more then a handful of users use then you'll need a pretty significant budget to keep it running without the fear of the government cracking down on you.