While in general we want slices to follow bytes and strings, I think we've learned over the years that the use of "left" and "right" is parochial and confusing. We should use "begin" and "end" or something similar.

While we are bike shedding, I feel the comments on the API suggest TrimLeading and TrimTrailing.

the *Func variants feel really close to a theoretical Filter

Changing the words "Left" and "Right" sounds fine to me, but I do have a preference for consistency with the "strings" and "bytes" package. Presumably we would introduce functions with the updated names there too?

the *Func variants feel really close to a theoretical Filter

It's actually more like DropWhile because it stops filtering after the first false return and then just returns the rest, even if it passes the filter.

I like slices.DeleteFunc because it takes the convoluted idiom for filtering a slice in place and makes it much simpler. If these return new slices, I'm not sure they meet the bar for simplifying complicated code you could just write some other way.