What determines the reversibility of cell of G0 phase in cell cycle?
The answer is quite complicated. One of the major issues in understanding what factors drive senescence vs quiescence has been the absence of a clear, uniform molecular definition of senescence. That said, multiple tumour suppressors, including p16 (and the other genes that occupy the INK/ARF locus) , as well as TP53, can modulate the expression of senescence associated transcriptional programmes. 

Additionally, epigenetic remodelling, including heterochromatinisation at E2F target genes (which are a major driver of cell cycle progression through the S phas) , is also known to be a feature of senescence. 

A very good starting point to survey the literature surrounding the topic is https://www.nature.com/articles/nrc3960?message-global=remove