FPSC conducts recruitment drives throughout the year. This includes CSS as well as General Recruitments. If you read the notices on FPSC's website, all general recruitment processes were delayed due to corona since March 2020. The recent delay notices declared that the recruitment processes (including MCQ test, descriptive test and interviews) will continue from mid of September 2020. This means that everything has been delayed by 6 months (due to halting of all recruitment processes from March to September).

They were able to declare CSS 2019 result because only a few interviews were left when the lockdown was imposed. Those few candidates were called for the interviews as soon as the lockdown was relaxed and so FPSC was able to declare the result of 2019 without much delay. CSS 2020 written result will also be announced on time because the processing of checking the papers was not affected by the lockdown. However, as far as CSS 2021 is concerned the papers will likely be delyed by 4-5 months. This is because FPSC has to first conduct all the MCQ test, describtive tests and interviews which were delayed due to COVID, not to mention the announcement of government to conduct a special CSS exam before the 2021 exam. This is just my opinion, I may be wrong.