That is the biggest issue with the modern ECU's and electronics we have put into classic cars. Most new cars have some small amount of parasitic drain but in theory they get driven regularly where our classic cars do not. A battery maintainer is pretty much a necessity. You can also utilize a battery disconnect which I recommend.
Not all maintainers or chargers are created equally. I use R2's for their digital readout but CTEK and others use the same pulse type charging technology aka (smart chargers). Learn about the difference here.
https://www.r2batterysaver.com/batte...earch-summary/
https://www.r2batterysaver.com/produ...attery-rescue/