As others have said it's a target plucked from the air by a government that has had to add 55,000 to HMRC to deal with post Brexit friction. The proper way to do it is to review functions and abandon any found to no longer add value but then invest in others. Examples are tired IT in DVLA, the Home Office and legacy benefits, or at least those not being rolled up into Universal Credit.
It's also being reported that the government has resuscitated plans from the Osborne era for further cuts to the Civil Service Compensation Scheme. This gives contractual redundancy pay much more favourable than required by statute. It was previously reformed during the Coalition so as to make it 'fit for another generation'. If they get away with it this time then the cost of losing those who take 'voluntary' redundancy will be significantly reduced.
I was made redundant in 2013 after 35 years under the coalition's reformed scheme. On voluntary terms I got early pension at 55. If I'd let it get to compulsory it would have been weekly pay * years of service but likely capped.
