In the referenced message, Roy said:
In a recent discussion with a company that owns a /16 and has it broken
down further, the statement was made that there are ISPs that filter
routes at /16 in what was traditional class B space. The example cited
was Verio. Verio web pages state they don't do this any more (the
filter is /21).
Is there anyone that still filters routes longer than /8 and /16 in the
traditional Class A and B space?
Assume this is the case, announce your largest aggregates, and reduce
the likelihood of ever having a problem in the first place.
This does not prevent you from advertising additional more-specifics
to entities who have agreed to accept them, nor does it prevent others
from filtering them. In the end, you still have connectivity, and only
those who wish to see the more-specifics, will.