Outlining some relevant points; others please edit and also update as conditions change.
- Is the crypto coin in question either of 1) a money (kesef, lit. silver) or 2) a "thing normally dealt-interest-for (kol-davar asher yisshakh)?
Deut. 23:
לֹא-תַשִּׁיךְ לְאָחִיךָ, נֶשֶׁךְ כֶּסֶף נֶשֶׁךְ אֹכֶל: נֶשֶׁךְ, כָּל-דָּבָר אֲשֶׁר יִשָּׁךְ.
20: Thou shalt not give interest to thy brother: interest of money, interest of victuals, interest of any thing that is [normally] dealt-interest-for.
לַנָּכְרִי תַשִּׁיךְ, וּלְאָחִיךָ לֹא תַשִּׁיךְ--לְמַעַן יְבָרֶכְךָ יְהוָה אֱלֹהֶיךָ, בְּכֹל מִשְׁלַח יָדֶךָ, עַל-הָאָרֶץ,
אֲשֶׁר-אַתָּה בָא-שָׁמָּה לְרִשְׁתָּהּ. {ס}
21 Unto a foreigner thou mayest deal out interest; but unto thy brother thou shalt not deal out interest; that the Hashem thy God may bless thee in all that thou puttest thy hand unto, on the land whither thou goest in to possess (inherit) it. {S}
- Rashi on Deut. 23
לא תשיך. אַזְהָרָה לַלֹּוֶה שֶׁלֹּא יִתֵּן רִבִּית לַמַּלְוֶה (עי' ספרי) (וְאַחַר כָּךְ אַזְהָרָה לַמַּלְוֶה אֶת כַּסְפְּךָ לֹא תִתֵּן לוֹ בְּנֶשֶׁךְ): לא תשיך
20: implies a prohibition addressed to the borrower that he should not pay interest to the creditor (cf. Sifrei Devarim 262:1; Bava Metzia 75b) [and afterwards (Leviticus 25:37) follows the prohibition addressed to the creditor, “thou shalt not give him thy money upon interest”].
לנכרי תשיך. וְלֹא לְאָחִיךָ, לָאו הַבָּא מִכְּלַל עֲשֵׂה, עֲשֵׂה, לַעֲבֹר עָלָיו בִּשְׁנֵי לָאוין וַעֲשֵׂה: לנכרי תשיך
21 UNTO AN ALIEN THOU MAYEST LEND UPON INTEREST (according to Rashi: TO AN ALIEN THOU MAYEST PAY INTEREST) — but not to thy brother. Such a prohibition which is not plainly stated but can only be drawn by inference from a positive command is itself regarded only as a positive command — so that one who pays interest to his brother transgresses two negative commands: לא תשיך in v. 20, ולאחיך לא תשיך in v. 21 and a positive command לנכרי תשיך — ולאחיך לא (cf. Sifrei Devarim 263:1; Bava Metzia 70b; also cf. Rashi on Deuteronomy 14:20).
Per answers given in this question: Lending Money and Inflation the consensus seems to be:
The [government recognized] currency is always considered to have a constant value. Any [pairing variations thereto] are attributed to fluctuations in the value of the merchandise [i.e. non-currency item], and not to changes in the value of the currency.
In point 2., which "government" is regarded? In June 2021 El Salvador became the first country to make Bitcoin (BTC) the legal currency, and was followed by several other Central American countries in doing the same.
In the USA, state law vs. federal may or may not be in play.
In June 2021 the Texas Department of Banking announced that state-chartered banks have the authority to provide custody or safekeeping services for virtual currencies, and Gov. Greg Abbot has a forthcoming "Virtual Currency Bill" that recognizes virtual currencies' legality, making Texas the second state after Wyoming to recognize blockchain and cryptocurrency.
- Following Point 2., Who (Ruven) pays to whom (Shimon) the interest? And in what country does Ruven live?
Therefore the simplest case would be if Ruven and Shimon lives in El Salvador; then Ruven paying out interest on and in BTC to Shimon would appear to be Ribbit. What if Shimon resides in a non-BTC land?
For your particular question, per celcius.network as of June 2021,
A. Celsius is registered in the UK.
B. Up to 80% of the revenue is paid out to the community. (So the remaining 20+% presumably going to expenses and management)
Presumably [at this time] the majority of the community (Celsius network) is outside of lands recognizing BTC as currency. So ...
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