Do I need to follow traditional cover letter style for applying for an academic position? I am applying for a research assistantship at a university, and I am required to upload a cover letter for submitting my application. I am not familiar with cover letter, since I'd only applied to student positions before, where I was required to upload an SOP and a CV, instead of a cover letter and a resume. This application still requires a CV, and not a resume, but instead of SOP, it requires a cover letter.

I looked for cover letter templates online, and it has the form of a letter (starting with "Dear..."), and "sender" and "recipient" addresses on top of the page, which sounds weird to me since it is not 1918 ad I am not posting it to them, I am uploading it to their application portal, and my address is also on top of my CV and I separately enter my address when I fill in the application form (and they know what their address is, so I'd say they probably don't need me to write it for them on top of my cover letter!). 

Is it just an outdated thing, or a convention that I need to follow? 

Also, given the position is academic, do you think is it ok if my letter is some sort of a hybrid of cover letter and SOP?!
Here is my opinion based on my rather limited experience:

In the English-speaking world (and probably elsewhere except, perhaps northern Europe) sticking to a traditional letter format is probably a good idea (as if it was being posted). Also it is good to have your contact info on all correspondence materials, especially your email address.

Keep the letter short - one page max.
Start the letter with a Re: (the position applying for) line.
Find out who the search committee chairperson is (phone or email the department secretary) and address your letter to that person with appropriate title (usually Dr. rather than Professor, but in Germany both, if applicable).

Tell them why you want the position, why you want to work at that university (do some homework) and why you are qualified - be enthusiastic.

If it is a research-oriented position (rather than just teaching) emphasise your research interests and how they connect with the faculty there. If just teaching a short paragraph would do.

Let them know that you are available for interview (with blackout dates if necessary) including a phone interview to start with, if desired.