I understood Acts 1:21-26 to be the requirement for the original "Twelve" apostles (eg, Matt 10:2, 5, Luke 9:1, 18:31, Acts 6:2, Mark 6:7, 9:35, 14:10, John 6:70, etc) but not a requirement for apostles more generally. The reason for this is as stated in previous answers:
- there appears to have been mores apostles after Paul including: Epaphroditus (Phil 2:25), Titus and other brothers (2 Cor 8:23), Andonicus and Junia (Rom 16:7).
- Paul instructed the Corinthian church to eagerly desire the spiritual gift of apostleship as per 1 Cor 12:28-31 -
And in the church God has appointed first of all apostles, second prophets, third teachers, then workers of miracles, and those with gifts of healing, helping, administration, and various tongues. Are all apostles? Are all prophets? Are all teachers? Do all work miracles? Do all have gifts of healing? Do all speak in tongues? Do all interpret? But eagerly desire the greater gifts.
According to this very passage, the greater gifts include, first and foremost, the gift of being an apostle - Paul cannot here mean that apostleship was no longer available!
Note further, that Paul was not counted among the "Twelve" but was still an apostle who clearly did not, and could not be an apostle by the definition in Acts 1:21-26 which lays out the requirement for the original "Twelve"
- accompanied us during all the time that the Lord Jesus went in and out among us, 22 beginning from the baptism of John until the day when he was taken up from us
- witness to his resurrection
Paul satisfied neither of these requirements and neither did Epaphroditus, Titus and other brothers and Andonicus and Junia, yet all were counted among the apostles more generally, because they were appointed and sent by God as was Paul, eg, 2 Cor 1:1, etc.