Every year until 2019 we are releasing the IRS values for the customers so they can upload them using procedure explained on How can I update my system with the new values of tables . Note 816164 - HR-PT: Procedimento de Carregamento tabela de IRS contains the new tables for 2020 so you can upload them into your system.
Very important before doing this on 2020 make sure you have note 2855074 - RPUTAXP0: Technical Enhancement - Part 2, vem delimitar a data final do ano anterior implemented on your system
NOTE: IRS Table T5P1P is a customizing table where the Customers can update he/she the tables but they still use their own data. From SAP we will evaluate if on 2020 we will release again the tables (not confirmed yet) on this note but this is a help for the partners and they are the resposible to update this tables.