Not sure what you mean by allowed. It’s a quirk of the way Unicode works. To “disallow” such text would be to artificially limit the way Unicode works.

G̵̖̺̩͔̫̬͇̲̙̙̘̹̀̋̚ư̸̧̨͔͍͔͇̜̹̻̼̤̳̤̥̠͚͈͓̹͙̻̎͗͋̽̈́́̊̄̀̓̌̍͊͒̓͆̔͌̉̂͘͜͠ͅe̸̝͕̦̖̭͚̣̰̖̗̬̳̜̳̬̍̌̊̕͜ͅs̵̛̪̻̯̀͆͂̀̏̓̀̅͐͊̽̓̎̒̓̀͊́͑͌́̏̈́s̸̨̧̥̺̗̗̣͍͓͈̮̙͔̻͍͕̫̩̐̈́̋̾̚͜ ̸̧̮̰̮̫͙͉̦͇͙̜̙͓̳̤͖͍̜͈͈͓͖̦̬̭͓̰́̐̅́̃̆ͅͅỵ̴̧̧̧̼̜͖͎͉͎͊͌̎̐̀̓̈͑̍̃͒̿͂̏̋̀͋͊̿̇̈́͊ǫ̴̰̺̈͒́͒̑̔̔̈̈̏̽͒̾̚͝ų̸̜̪̼̮̥͉̅̿͋̿͗̋͑͗̈̀̊̓̒̈́͌̋̆͘̕̕͘͝͝’̴̧̧̨̡̨̫̱̣͇̠̞̘̭̝͔̪̝̩̮̯͕̭̠̪̩̜͗̈́̀̋̽͘͠ͅͅv̷̧̧̨̱̲̹͎͈̲̙̼̬͈͖͍͓̻̞̥̖̮͙̘̣̙̪̭̽̉̃̑͗͂͊̋͆̏̈͗̑͊̕͠ͅȩ̵̛̮̙̹̳͕̝̲̼̣̠̭̬̥͉̲̭̲̦͈̗̜̳̙̎̎̐̿̇͑̀͆̈́́̽̆̾̍̕̕͝͠ͅͅ ̸̧̢̛͕̠̲̗̜̻̹̥̻̜̲͕̦̲̂̽͐͗͑͐̎́͘ͅn̶̛̤̤͚͙̑̆̾̋̓̊̆̀͐̾̑͝ͅe̵͔̜̘̫̘͍̫̯̘̞̺̪̝̐̆͘v̷͕͙̩̬̩͎̮͚̬͔̯͙̩̦͎̙̫̮̪̝́͐̅̓̈́͂̌͐̊͑̾̆̍͑̈̄͒͆̊̓͗͊̅͊͘͜͠͝è̴̡͈̥̙̙͎̹̳͙̙̻͜r̸̖̻̦͎̣͓̤̙̬͎̱̟̩̺̰̥͓͍̖͎͋̾̄̓͋͂̚ ̵̡̨͍̪͚̯̜̖̫̲̓̅͊̈́̃̏̈́͐̿͆͘h̵̨̡͉̼̣͕͈̙̝̬̼̬̯̖̣̪̟̩̦̺̥̪́̒͂̃̈́̀͑͠ͅe̵̢̳͖͖͉̲̹̣̭̺̪̲̩̥̰̬̟͉̪̊̾̉̀͗̏̀̀͆͂̈̉̆̀͆̍̕͘̕ͅḁ̶̧͉͈̝͇̣̦̜̺̹̦͚̰̲̺̱͔̱͖̻̮̘̣̔͂ͅr̴̡̢̦̱̻͈̻͈͎̻͖͙̥͉̹̙͎͓̯͉̖̗͓͉̅͜͝d̵͖̺̜̫̰͍̰̜̫̤̗̻̘͔͓̜̱͚̗̗̩͒̏̀̇̿̀͘͝ͅͅ ̶͙͈̩̱͈̼̥̳̥̳͉̩̜̠̦̗͈̾̓̒̆̀̾̓̄̂̈̓̓̕͘͜ō̵̡̧̻̤̮͇͔͍̹͚̻̱̗̤̬̻̫͚̲̥̦̽̽̍̏̄̈́̈͊̓̈́̉͆͐͌͒̌̊̄̐̍̽͐͛̉͆̚͘͜ͅf̷̡̼̫̰̖̻͓̜̤͎͖͕̙͈̠̫̮̝͇̗̩̼̬͂̎͌͛̀̓̔͋̐̊̕͜͜͝ͅ ̴̧̡̧̢̛͔̬͔͍͓͖̳̮̘̮̺͔̗̩̫̘̗̥̫̅̈́͆͛͐͆̈͒͊͋̽̑̈́͜͜ͅz̸̘̜̈́̓͐̐̉̎́̔̀̋̈̒͛̏̃̓̕̕a̴̧̧̛̙̩̮̙̝͇̖͎̦̳̩̮͍̣̤̩̙͔̼̮̼͍̱̬̻̾̂͋̓͗͋̓̂̿͛̉͌̈́̈́̾̚͜ͅͅļ̸̡̛͍͙̗̳̦̝̯͙̅̋̀̉́̃̒͋͛̒̍̓̀̾̕̚͜͝g̴̢̢̧̧̛̫̹̳͓͕̪̺͚̩͓̱̦͍̘̮̖̜̜̳̪̞͎̼̮̖̾̔̆̾̉̅ǫ̸̧̨̡͙̹̙̲̻̤̱̺̜̠̞̫͕̮̮͙͙̙̫̙͒̆͒̔͐̈̇̿͋̽̋̔̒͆͆͊̌̌̀̒̂̉͘̚̕ͅ

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