In my customers situation this is quite bad and we have to think about replacing 5110s or redisign with vxlan, if they are not going to support NSSU... otherwise the switches will be stuck on an old version cause the customer can't afford the downtime.
There are no plans that I am aware of for NSSU support with VCF. Attached is a "potential" alternative minimum downtime procedure for upgrading a VCF. This is NOT Juniper certified or JTAC approved, but should work if followed correctly and precisely.
As for NSSU for VC, full-support for QFX5K series should be coming in 19.3R1/19.4R1. This will also include EX4600/EX4650, not sure about other EX Access, but suspect yes.