Yes, the underlying S3 texture compression algorithm was patented in the US in the late 90s. The last, relevant patent expired in 2018[1]

Direct3D called its variants DXTn, later rename to BCn. From what I recall, Microsoft had some sort of patent licensing deal that implicitly allowed Direct3D implementers to support their formats.

OpenGL had an extension called GL_EXT_texture_compression_S3TC[2].

Under "IP Status" the extension specification explicitly warns that even if you are e.g. shipping graphics cards with Direct3D drivers, supporting S3TC, you may not legally be able to just turn that feature on in your OpenGL driver.

[1] https://en.wikipedia.org/wiki/S3_Texture_Compression#Patent

[2] https://registry.khronos.org/OpenGL/extensions/EXT/EXT_textu...