Are more teachers leaving the profession? Well it depends upon how you want to measure the outflow: by percentage or by actual numbers. The DfE helpfully provides the base number of new entrants and then uses percentages to show the degree of wastage from the profession over time. However, the actual number entering the profession each year fluctuates, as recruitment flows and ebbs according to how teaching is seen as a career. As a result a lower percentage remaining in the profession can still mean a larger number remaining in teaching when comparing retention over a particular period of service, but for different years.
The two tables demonstrate this quite clearly.

The source of the percentages is the DfE evidence to the STRB, published in January 2018.
Although the percentage remaining after one year of service has been on a downward path, the actual number been increasing due to more entrants into the profession. Sadly, the data for 2019, when it appears in 2020, will probably show a dip due to the poor recruitment into training in 2017.
What really matters, and isn’t clear from this data, is the breakdown between primary and secondary sectors and for the different subjects within the secondary sector. This is because those that remain must provide the majority of the new leaders every year. By year six, if there are half remaining in the primary sector that is between 7,500-8,000 teachers per cohort. With around 1,200-1,500 school leadership vacancies per cohort that means around 20% of teachers remaining by their sixth year of service might expect to be in a leadership position at some point in their careers.
Finally, it isn’t clear whether the DfE adds in late first time entrants to their original cohort or just ignores their existence. Hopefully, their contribution is recognised within the data, but not made explicit.