It is a past employer actually.Is it a current or past employment?
If you are still working in the same workplace, you will need a recent reference letter but if you stopped working there since 2018, you don't need to get a new one. IRCC would contact the employer to verify your claims if they have any doubt.
I left the job with that employer in 2015. When I initially started planning for immigration in 2018, I reached out my employer (basically the supervisor I was working with) and requested for the reference letter. This is because I didn't want to delay it keeping in view the excuses coined by the employers sometimes as well as the management change that might take place any time. After some struggle, I was able to obtain the reference letter meeting the IRCC requirements.
But I am worried if I need to submit the profile now, would that reference letter dated 2018 be still valid? Because getting it renewed might be a long hassle.