the term broadly can be used to describe poor imitations of retro aesthetics without acknowledgement for the fundamental qualities of why something retro is better.
For example, a shitty tranny-made unity game that emulates the psx style is retroslop, namely because its not well-made like a beloved older game nor does the chunky art style come from hardware limitations being worked around, and instead is just chosen to merely look that way.
Another example, a shitty tranny-made neocities website that emulates the look of a geocities website is retroslop, especially if instead of using IRC or at least XMPP, they link to their discord.com account (a gross modern manifestation)
most retroslop is simply not retro at all and merely superficially emulates an older style to appeal to nostalgia or some sense of oldfag street cred one doesn't actually have.
It's just the modern version of those steampunk soyboys merely emulating the vague looks of the victorian era while not at all mimicking their way of life or values.