As of June 2021 BankID requires Android 6 or newer. If your device cannot be updated you cannot run BankID on it. More info here https://support.bankid.com/sv/fragor-svar/tekniska-fragor/varfoer-har-ni-aviserat-att-bankid-appen-kommer-att-sluta-stoedja-android-5-etc
BankID Security App is part of Mobile BankID - an electronic personal identification system for mobile phones and pads used by various service providers.
Mobile BankID is issued by Danske Bank, Handelsbanken, ICA Banken, Länsförsäkringar, Nordea, SEB, Skandia, Sparbanken Syd, Swedbank and Ålandsbanken.
More information (in Swedish) for developers of electronic services can be found at www.bankid.com/rp/info.