very old injury causing interior wood to eventually rot and lose all structural integrity ... you sound lucky you werent injured ...

what in the world is all the fishing line and electric wire for ...

i see evidence of poor pruning.. leaving long stubs ... and by the trunk size presume this is a very old tree ... i would suggest you either:

1... get rid of it.. and be done with it ...

2... leave it.. but plant a new tree.. so as this one breaks down over the next few years.. [tree time] .... you already have a new tree well on its way to replace it ...

ken

ps: if you have kids around of climbing age .... it might be best to be done with it now ...