Warning: Text under spoiler tab contains NSFW terms
It's satire to the whole absurd censorship rules in the anime industry (see this video to know more on these absurd rules). I did some digging and found exactly what I needed:
Hand Holding is another act that is used in a similar manner to Consensual Sex in the Missionary Position. Threads with hand holding as the topic will often receive satirical replies usually criticizing how the original poster is sick or lewd. Related images of this at time feature anime characters with unnecessary censorship photoshopped over the hands to add further lewdness. (source)
There are many reddit post discussing this meme (see refs.) all agreeing on the satirical take but sometimes there are some specific but very absurd reasons:
Many "child-friendly" books explained baby-making as "when your parents love each other very much by holding hands and/or hugging very tight, mommy gets pregnant". This, however was considered too lewd for children and hence had to censor them. (See Naisho no Tsubomi)
It's a mix between a generalized joke about the Japanese censoring genitalia in doujins as well as the fact that Romance series often end on hand holding and often without even a kiss, people started joking that handholding was too lewd for network TV. This joke was taken so seriously that even handholding was censored in hentai and borderline hentai giving up a notion that if you want to censor anything, censor everything (e.g. Dakara Boku wa, H ga Dekinai). Therefore, holding hands which is one of the purest acts of a romantic relationship is censored as if it's a filthy act just like censoring porn/guro making it somewhat hilarious.
- What is this censored hand holding thing in Anime? -reddit post
- Why would they pixel that? - reddit post
- What started the censored holding hands meme? -reddit post
Bonus: Not only holding hands were censored as a joke, even the thoughts of holding hands and cuddling were satirized to be impure. This is a scene from Oremonogatari:
This image is actually a part of Tumblr post where the poster made a cheeky remark (Warning: Strong language).