HD masters actually first started appearing in the mid to late 90’s but of course these were very very low res and by no means measure up today. WB did a number of high scale scans for several of the classic titles early on but it was mainly Sony who led the front in new or spruced up HD or quasi-HD scans. After a certain point they released everything as a “mastered in High Definition” title.

It wasn’t until the advent of BD on the horizon that 4K really began to take off. But still the vast majority of things are 2K scans/workflows and most films are only finished at 2K.