The problem is that the ~~ at the end are not separated from the link. It may be difficult to decide if they are part of the link or part of formatting.
__https://meta.discourse.org/t/horizon-theme/360486/1__ doesn’t work either __https://meta.discourse.org/t/horizon-theme/360486/1__
You can use one of these instead
~~[Horizon Theme](https://meta.discourse.org/t/horizon-theme/360486/1)~~
[s]https://meta.discourse.org/t/horizon-theme/360486/1 [/s]
<s>https://meta.discourse.org/t/horizon-theme/360486/1 </s>