The rule has always been that people with a residency permit can enter Germany. This is AFAIK pretty standard as far as it comes to Covid-related entry requirements around the world.
Deutsche Staatsangehörige sind von den Einschränkungen nicht betroffen.
Auch Unionsbürger und Staatsangehörige des Vereinigten Königreichs, Liechtensteins, der Schweiz, Norwegens und Islands und ihre Familienangehörigen der Kernfamilie (Ehepartner, minderjährige ledige Kinder, Eltern minderjähriger Kinder) sind von den Einreisebeschränkungen ausgenommen.
Das gleiche gilt für Drittstaatsangehörige mit einem bestehenden längerfristigen Aufenthaltsrecht in einem EU- oder Schengenstaat oder dem Vereinigten Königreich (Aufenthaltstitel oder längerfristiges Visum) und ihre Familienangehörigen der Kernfamilie.
(from https://www.bmi.bund.de/SharedDocs/faqs/DE/themen/bevoelkerungsschutz/coronavirus/coronavirus-faqs.html => "Welche Reisebeschränkungen gibt es im außereuropäischen Luft- und Seeverkehr?")
Your Fiktionsbescheinigung is probably due to § 81 Abs. 4 AufenthG, which means it is essentially an Aufenthaltstitel.
So once you make it to Germany on a plane, you should be able to enter. The more difficult problem may be getting the airport personnel in India to allow you to get on the plane, due to the unfamiliar combination of travel restrictions for non-residency permit holders and you only having a Fiktionsbescheinigung. Might be a good idea to check with the airline first, get some written reply and arrive early at the airport for your return trip in case there are problems.
Re. possible quarantine: My impression is that countries that are trying to eradicate Covid-19 have very strict quarantine rules (i.e. quarantine "hotels" that you cannot leave and have to pay for out of your own pocket). Sometimes these rules create a bottle neck because quarantine capacities are limited and so only a certain number of people can enter the country per week (e.g. in Australia, but I am familiar with at least one other country with the same problem)
Quarantine requirements in countries that do not fall into that category are considerably more lenient. Germany currently falls into the category of countries that do not really attempt to bring Covid cases to zero, and quarantine requirements are not very strict. I do not think this will change until January, but you might want to follow the news. And also note that there is a possibility that India might tighten some rules and air travel gets more limited as a result.
P.S. there is also a possibility that new entry requirements are added. One such requirement is already in force (registration at https://www.einreiseanmeldung.de/ ) but it is easy to imagine more might follow. This is also something to watch and keep in mind. E.g. if they, say, introduce a requirement for a negative Covid test before you can board the plane, you would have to prepare accordingly
Another point to keep in mind: A Fiktionsbescheinigung is quite short-term. If somehow you do not make it back to Germany in time (and the probability of something like this happening is much higher than normal at the moment) things might get complicated.