The Russian had a frustrating rookie season in 2018, spending 21 races with Williams and only scoring a single point throughout the year. The point came because of a post-race disqualification for Romain Grosjean at the Italian Grand Prix as well, meaning he never actually finished in the top 10 for the Grove-based outfit despite having some very good drives throughout the year and beating teammate Lance Stroll in the qualifying battle.
Sirotkin, still only 23 years old, saw his financial backers SMP pull funding for his F1 seat after the season. He accepted a role as Renault reserve driver, stepping in in case either Nico Hülkenberg or Daniel Ricciardo are unfit to drive.
McLaren have now signed the Russian up for the same role at the Woking-based team as well. Renault will still have preference if he's needed at both teams for some reason, but this scenario is very unlikely.