I have this xsl:param at the top of my XSLT program:

<xsl:param name="ICAO" />

Is there any benefit to adding this "as" clause:

<xsl:param name="ICAO" as="xs:string"/>

That "as" clause says the ICAO param is a string value. Well, everything is

a string. These are all strings -- '123', 'true', 'http://www.example.com',
'01101' -- all of which should not be used to populate the ICAO param. It
seems to me that adding the "as" clause provides zero benefits. In fact, it
has disadvantages: it makes the code more verbose, it requires more typing. Do
you agree that adding the "as" clause provides no benefits?

Bonus question: The value to populate the ICAO param should be a 4-letter,

uppercase value, such as KBOS. Is there anyway to express that
requirement/constraint in the "as" clause?

Perhaps in XSLT 4
   as="enum('KBOS')"
Of course an enumeration requires you to be able to list the possible
values.

In schema-aware XSLT 2 or later I think you might be able to import a
schema that defines a type with some restrictions so that some of your
constraints are ensured and use that type in the "as" attribute.