acro-package: Always use long form for defined acronym?

The solution is to use my AFTERNOTE above together with the exclude-classes option to the printacronyms command. I have to include the field class = dummy in the acronym definition (any name for the class will do, it doesn't need to be dummy).

It's not elegant, but it works. Better solutions are welcome.

All acronyms can be set to give their long form by setting acro_use:n which check the acronyms used status and then picks long/short/first form accordingly to instead always fetch the long form with acro_long:n

cs_set_eq:NN acro_use:n acro_long:n

This won't affect calls for the short or alt forms, they will still appear as the short/alt forms as internally they use acro_short:n and acro_alt:n but these can also be set to call the acro_long:n form if so desired.

ExplSyntaxOn
cs_set_eq:NN acro_use:n acro_long:n
cs_set_eq:NN acro_short:n acro_long:n
cs_set_eq:NN acro_alt:n acro_long:n
ExplSyntaxOff

should essentially turn off all ac-type macros and make them expand to the long form only.

If looking to only disable it for a set of acronyms, say those identified with class = maybelongonly then one can modify the acro_use:n to insert a check of the acronyms class and accordingly print it as normal (for acro) or print the long form only. To check the class acro_get_property:nn is used, which was introduced in v2.7

documentclass{article}
usepackage{acro}

DeclareAcronym{ecm}{
  short = ECM ,
  long  = Electro Chemical Machining,
  class = maybelongonly,
}
DeclareAcronym{adc}{
  short = ADC ,
  long  = Analog-to-Digital-Converter
}

ExplSyntaxOn
tl_new:N l__acro_longonly_tl
tl_set:Nn l__acro_longonly_tl { maybelongonly }

cs_set_eq:NN acro_useold:n acro_use:n
cs_set:Npn acro_use:n #1 {
    acro_get_property:nn { #1 } { class }
    tl_if_eq:NNTF l__acro_class_tl l__acro_longonly_tl {
        acro_long:n { #1 }
    }{
        acro_useold:n { #1 }
    }
}
ExplSyntaxOff

begin{document}
ac{ecm}

ac{ecm}

acp{adc}

Ac{ecm}

acp{ecm}

Iac{adc}
end{document}

this will ensure acp{<key>} prints always printsthe long version with the appropriate plural for acronyms in the appropriate class. In order to catch any explicit calls for the short or alt versions (acs, aca etc.) then acro_short:n and acro_alt:n should be modified similarly to acro_use:n.

These acronyms can be removed from the list by giving printacronyms the key exclude-classes=maybelongonly.

Starting with v3.4 (and in analogy to the option and the property first-style) acro has the option subsequent-style which sets the style for all appearances after the first for all acronyms, to be set with acsetup{} and initially set to short. It also has the property with the same name which sets the same for an individual acronym (and which of course takes precedence over the option):

documentclass{article}
usepackage{acro}[2020/12/25]% v3.4 or newer

DeclareAcronym{aa}{
  short = a ,
  long = the letter a
}

DeclareAcronym{bb}{
  short = b ,
  long = the letter b ,
  subsequent-style = long
}

begin{document}

First: texttt{aa}: ac{aa}, texttt{bb}:  ac{bb} par
Next: texttt{aa}: ac{aa}, texttt{bb}:  ac{bb}

end{document}