The correct URL should be https://taginfo.openstreetmap.org/keys/garden%3Astyle.

It is trying to encode the colon character it as its numerical value. You can do this with any character: https://taginfo.openstreetmap.org/%6b%65%79%73/%67%61%72%64%65%6e%3a%73%74%79%6c%65.

It is mostly used for special characters like spaces and Unicode characters. However, the colon character is allowed in URLs so it would be possible to just use https://taginfo.openstreetmap.org/keys/garden:style.

The %252A is what you get when you encode the URL twice.

“:” → (encoding : as 2A) → “%2A” → (encoding % as 25) → “%252A”