before the junk data of appended data after signature
there was op_return
but this too was not used as a data exploit in 2009-2014
https://bitcoin.org/en/release/v0.9.0#rebranding-to-bitcoin-core

back then core pretended to be against wanting bitcoin to be used as a junk data exploit, yet, stupidly opened the exploit to allow 40byte(later 80) and then in 2017 allowing uto 4mb using a different opcode

it has never "always been possible"
it has only been possible since core jumped in and changed things