Historically the bitcoin consensus rules about things that aren't checked have been loose (they still are) for example your output script can be literary anything like an invalid script or any arbitrary data of any size like a picture or something like that.
This is a good thing because it allows future expansion through soft forks but also it could lead to abuse of the system and spam attacks.

Over the years to prevent such abuse (eg. placing arbitrary junk in your scriptpub) the standard rules were introduced where the nodes rejected such non-standard transactions and it successfully prevented spam.

When it comes to SegWit, the rules are flexible in many places that allows future expansions but only some of them were prevented as non-standard while others fell through the cracks and helped creation of attacks such as Ordinals.