I really don't think supporting the OP in this way is helpful at all. As someone else said above, there are lots of other Grandparents on this thread suggesting modern parents and their 'rules' are ridiculous but, in reality, there are very few things that were asked and the majority of modern parents on this thread feel the same.
We have lived this experience ourselves. My in-laws were cut off (with very good reason!) by BOTH husband and I. The boundary crossing by MIL (and other issues which I won't discuss here) nearly broke our relationship and husband was sick of the same patterns repeating from his childhood. However, I'm sure MIL found her echo chamber and I'm still painted as the 'evil daughter in law' who took away golden boy and her grandchildren, whereas in reality husband was the one to make the shift and put his foot down. Daughter in laws are the easy scapegoat rather than dealing with the issues which, from what her son had said to her in conversation, seem ongoing perhaps even from beyond this relationship.
If OP wants a relationship with her son and grandchild in future, she needs to listen to what he has said, respect it, learn from it and then she'll most likely be given a chance.
My MIL refused to, so she hasn't been in our lives. It has been wonderfully peaceful for us but she has lost out because she refused to accept or deal with the fact she caused so many issues in my husband's life, refused to change her behaviour.
I think the turning point for my husband was experiencing how my family treat him, always with kindness and support, never any criticism, which showed him that how his mother treated him wasn't right. It just shone a light onto behaviours he had accepted as normal but in reality were really dysfunctional.
Maybe this is a similar situation as the broken rules seem to be just the tip of the iceberg from what the son said to OP.
Also, others have suggested thay in the future they will 'change their tune when they want childcare'...that won't happen either. We never went back, we've always been self sufficient, so please don't bank on them coming back for that. I think this is giving the OP a false sense of what might transpire.
OP, I am incredibly sorry that you lost your husband, it must have been a difficult few years but you DON'T have to lose your son, grandchild and daughter in law. Please perhaps seek a therapist who may be able to support you, not only through your grief but also through navigating how you could perhaps find your way back to your family if you respect their wishes. I am sure they would embrace you back warmly if you put in the work to not be critical of them and respect what boundaries they have in place for their child.
I just worry that others goading you/ supporting your side aren't helping and will potentially make things worse if you follow that advice.
If my MIL had done the work and listened to my husband then I think our lives would have looked very different. As it stands, she hasn't been in our life at all as she refused to engage. It doesn't have to be like this for the OP.
I really hope things work out for you OP.