As mentioned by this blog here and here, the Chinese security law makes “crimes” of subversion, terrorism, and co-operation with foreigners punishable by a minimum sentence of three years. The law is “designed” so broadly and vaguely to serve the communist regime to use it to quash freedoms of speech and assembly, political dissent and an independent judiciary. Features of Hong Kong that were taken for granted allowed in Hong Kong for decades, unlike in mainland China.
On the very first day the law entered into force, the police had arrested more than 300 people , nine of which were arrested specifically for violating the new security law. Oce Hong Kong is brought to its knees, the Communist regime will go for Taiwan where citizens still enjoy their freedom and can exercise their civil rights without fear of being arrested.
In the wake of Communist China take-over, getting arrested for posting political content online becomes highly likely.
Source: V-Dem https://www.v-dem.net/en/ Note: Arrests for political content. The indicator ranges from 0 to 3, where 0 indicates that it is extremely likely to get arrested for posting critical political content online, whereas 3 indicates that it is extremely unlikely.