After the post regarding being asked to delete social media posts by a firm you are due to join, I thought I might just explain why/how this could happen. I am not surprised by this - it is something I myself have experienced (albeit a long time ago now).
Most firms (and many other employers/organisations) will have a social media policy as well as policies around generally bringing the organisation's name into disrepute. If someone's social media does this, even if it is in the past and from some time ago, the firm has the right to ask you to delete it. Whether you choose to or not is obviously up to you - but if it breaches a policy, be mindful that your employment contract is likely going to have a term in it that you need to abide by your employer's policies etc.
If you are going to start with the firm or have already associated yourself with the firm (e.g. maybe adding on LinkedIn you are a future vac schemer or trainee), you are connected to the firm and your personal brand is associated with their corporate one. Because of this, you need to be aware of this and think about your digital footprint, no matter how old it is. Take it from someone who has to consider this with each post I put on here because I use my full name.
That makes it sound like you have to be super cautious of everything you put out into a public space, whether it be online or in the real world. Reality is the vast majority of stuff is not going to breach such policies - those that usually do are pretty obvious. However, political views can often be a grey area that is harder to navigate, especially at times like now when political tensions are very high.
I recommend everyone is mindful of this and considers their personal brand, especially when your are clearly identifiable from your content.