ARROW-5190, the R package's dependency on tibble has been moved from Imports to Suggests. However, this signature of read_parquet(), read_delim_arrow(), read_json_arrow(), and read_feather() all still have an argument called "as_tibble".
Per this comment the argument "as_tibble" is a misnomer. If that argument is given a TRUE, those functions will return a data.frame, not a tibble. I agree with Neal's comment...I think that instead of changing return type based on a boolean, it would be better for all the read_* functions to return a data.frame and for other types of operations to get their own functions.
I think it's important to consider this type of user-facing breaking change now while the R package's version is still 0.x.
As always, thanks for hearing me out!