I was interested in this as well when my first lymphocyte count came back at 710 /μL ("grade 2" lymphopenia). This was halfway thru RT and I asked the doctor on site about it. He told me, it is so common they don't test for it.

In a new test yesterday it was 490 (borderline Grade 2/3). My doctor told me there was nothing to worry about at 710, anything above 500 is fine 🙂

From a paper on the subject:
Radiation-induced lymphopenia (RIL) has long been observed in radiation therapy patients (1–3) and develops in up to ~70% of patients undergoing external beam radiation therapy (4–8).

I found this in one study:
Lymphocyte LD50 (lethal dose required to reduce the surviving fraction of lymphocytes by 50%) is 2 Gy and LD90 (lethal dose required to reduce the surviving fraction of lymphocytes by 90%) is 3 Gy (Nakamura et al., 1990).
Given I had 65 Gy dose, it is hardly surprising a few were killed 🙂

You may be interested in this Danish study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6211632. In here you can see the elevated risk of infection (Fig 4) and death from infection (Fig 9) vs lymphocyte count.

In my case it seems I have a 1.8 times higher chance of infection and 2.5 times higher chance of death once infected compared with what I had before. (this is my interpretation of the graphs, please correct me if I am wrong)

Another older retrospective study found no increased risk (but the Danish study was probably of higher quality). I can find the older study if you are interested.

Unfortunately, from what I understand, it will take a few years to get back to normal, so I plan to be a bit careful.